HP 7936 Support Manual page 64

Functional Description
7936 and 7937
asserted with the positive-going edge of SOS-L, at
the end of the sector in which the fault was
detected.
000-007
DDS-H
SEF-L
SOS-L
WHO-L
DISC DATA BUS
Disc Data Bus
Disc Data Strobe
Start ECC Field
Start of Sector
Write Holdoff
Disc data information exchanged between the
read/write controller and the DMA/ECC circuitry
in controller PCA-A6 is passed over the Disc Data
Bus. The bus signals function as follows:
Disc Data Bus (000-007) is an 8-~it bidirectional
bus interconnecting the read/write controller and
the DMA/ECC. The bus output is enabled when
the read/write controller is commanded to a read
mode and Start of Sector (SOS-L) goes low, until
SOS-L
goes high again.
Data Strobe (DDS-H), normally low, is generated
by the read/write controller. Data is transferred
between the read/write controller and the
DMA/ECC on the negative-going edge of the
DDS-H. During a normal sector of read, there will
be 276 low-high-low transitions of DDS-H. During
a normal sector of write, there will be 278 low-
high-low transitions due to two dummy bytes be-
tween the CRC and ECC fields.
Start ECC Field (SEF-L) allows the controller in
PCA-A6 to switch data paths from the data source
driving the bus to the ECC generator.
Start of Sector (SOS-L), originating in the servo
system is used in the Disc Data Bus as a handshake
line. SOS-L goes high for the duration of the
sampled servo field. The falling and rising edges of
the signal determine the beginning and end of the
data field.
Write Holdoff (WHO-L) can be asserted by the
DMA to inhibit a write sector. If asserted prior to
the falling edge of SOS-L, it will override a com-
mand from the controller, and no read or write
will take place.
5-30
5-94. Servo Signals. The read/write controller
receives the following signals from the servo
system:
SOS-L
WCLK-L
SVER-L
Start of Sector
Write Clock
Servo Error
5-95. Read/Write Control Signals. The signals
which the read/write controller exchanges with the
read/write system electronics are as detailed below:
INPUTS
CURUS-H
PARER-H
RDATA
RDCLK-H
Current Unsafe
Parity Error
Read Data
Read Clock
OUTPUTS
WDATA-H
WGTO-L
RDEN-H
PAREN-H
HDS1-H, HDS2-H
CHIPO-H thru CHIP3-H
DQUAL-L
WCSO, WCS1
Write Data
Write Gate Open
Read Enable
Parity Error Enable
Head Select 1, 2
Chip Select, 0 thru 3
Qualification Level
Write Current Select 0,1
The functions of the servo and read/write signals
noted above are summarized in table 5-1.
5-96. Control Section. The control section of the
read/write controller contains four registers: a
command register, a operation register, an address
register, and a status register (with associated fault
detect logic.)
The control section can be commanded at any
time by controller PCA-A6 to carry out a read or
write. The command from PCA-A6, which can
occur at any time in the sector, is stored in the
command register and is transferred to the opera-
tion register on the rising edge of Start of Sector
(SOS-L). Every sector must be commanded in-
dependently - the controller cannot be commanded
to read or write a number of sectors. Once the sec-
tor read or write operation is completed, the
controller reverts back to an idle state, unless